1. Field of the Invention
The present invention relates to hearing aids. More specifically, it relates to a hearing aid having a time-domain audio codec for decoding and encoding digital audio signals. The invention further relates to a method of decoding and encoding audio signals.
A hearing aid is embodied as a small, wearable unit comprising one or more microphones, a signal processor, and means for acoustically reproducing sound signals. A hearing aid may additionally comprise means for receiving, processing and reproducing sound signals from other sources, such as a telecoil or an FM receiver. In order to alleviate a hearing loss of a user, the signal processor of the hearing aid is configured to amplify selected frequency bands based on a prerecorded audiogram of the user's hearing loss. For flexibility reasons, the signal processor is preferably a digital signal processor.
2. The Prior Art
Modern day hearing aids are typically equipped with means for one- or two-way wireless communication, i.e. radio communication. Such wireless communication may carry sound signals, such as speech, suitable for being transmitted to and from the hearing aid in a digital form, e.g. between two hearing aids or between a hearing aid and another device. In such radio communication, there is a desire for keeping the transmission bit rate as low as possible, one of the reasons for this being that an increase in bandwidth of a radio communication leads to an increased power consumption, which, in turn, is undesired in a hearing aid.
One way to reduce the bit rate in a digital audio signal is to encode and decode the signals using an encoder/decoder unit or processor, commonly referred to as a codec, implemented as a combination of software and more or less dedicated hardware. However, such reduction of the bit rate comes at a cost.
One attempt to reduce the bandwidth and the delay time is described in the article: ‘A Low-Delay CELP Coder for the CCITT 16 kb/s Speech Coding Standard’, Juin-Hwey Chen et al, IEEE JOURNAL ON SELECTED AREAS IN COMMUNICATIONS, Vol. 10, No. 5, June 1992. The audio bandwidth, reproduction quality and computational complexity described in that article, however, do not meet the needs in a hearing aid.